Single piston activation of a planetary transmission
TECHNICAL FIELD 
     The present invention relates to transmissions, and particularly to the mechanisms for activating the brake and/or clutch used to control a planetary transmission. 
     BACKGROUND OF THE INVENTION 
     A typical planetary transmission has a sun gear, a ring gear and a planetary carrier with a plurality of planetary gears rotatably mounted thereon, with the planetary gears engaging the ring gear and the sun gear. It is known in the art to connect to input and output of the planetary to varying combinations of the sun gear, the planetary carrier, or the ring gear, depending upon the precise transmission characteristics desired. The typical planetary is shifted between different gear ratios by using a clutch to connect pairs of the sun gear, planetary carrier, or ring gear against relative rotation, and/or by using a brake to fix one of the sun gear, planetary carrier or ring gear against rotation relative to the transmission housing. 
     When both a clutch and a brake are used, the most common control mechanism is a spring-biased hydraulic piston to control the brake and a separate spring-biased hydraulic piston to control the clutch. Depending upon the exact characteristics desired, the springs for the pistons can either bias the corresponding components into engagement or out of engagement, with hydraulic fluid then applied to the piston to disengage or engage the component, as desired. 
     In many situations, the brake and clutch should never be engaged simultaneously. This means that the timing of application of hydraulic fluid to the pistons for the two control mechanisms must be carefully controlled. This poses the problem that the control valve structure for these fluids must be very carefully designed and manufactured to ensure that this timing is correct. 
     SUMMARY OF THE INVENTION 
     It therefore is an object of the present invention to provide a mechanism which can activate only one or the other of the brake and clutch of a planetary transmission at a time, without requiring careful design and manufacture of the control valves for the structure. 
     This object is accomplished according to the present invention by using a single piston to control both the clutch and brake. The piston is spring-biased to engage one of the components and disengage the second component, and hydraulically activated to disengage the first component and engage the second component. Which component is spring-engaged and which is hydraulically-engaged will depend upon the exact shifting characteristics desired in the particular application. 
     A convenient structure for this type of piston uses an annular plate positioned adjacent the planetary, with one of the shift components, e.g., the brake, located at a radially outward portion of the first side of the annular plate, and the other component, e.g., the clutch, located at a radially inward portion of the second side of the annular plate. A spring, e.g., a Belleville spring, can be positioned at a radially outward location on the second side of the plate, and a hydraulically activated piston can be positioned at the radially inward side of the first side of the plate, so that the actions of the spring and piston oppose each other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single drawing illustrates a typical structure for a planetary gear having a single piston actuation according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The figure illustrates a preferred embodiment of the present invention. In this particular embodiment, the planetary transmission 10 is driven via input shaft 12, which is connected to the planetary carrier 14. The planetary carrier 14 has a plurality of planetary gears 16, illustrated here as double gears, rotatably mounted thereto. The planetary gears 16 engage the ring gear 18 and sun gear 20 in the usual manner. Power is drawn from the planetary transmission 10 by output shaft 22, which is connected to the ring gear 18. 
     The sun gear 20 has a sun gear extension 24 which is provided with an annular slot 26. A brake pad 28 fixed to the transmission housing 30 extends into the annular slot 26 at the radially outward portion thereof. Similarly, a clutch plate 32 extends into the radially inward portion of the annular slot 26 and is fixed by splines 34 for rotation with the input shaft 12. An annular plate 36 is positioned in the annular slot 26 between the brake pad 28 and clutch plate 32. Annular plate 36 has at least one opening 38 formed therein through which a portion 40 of the sun gear extension 24 extends to prevent significant relative rotation of the annular plate 36 relative to the sun gear extension 24. The sun gear extension 24 is formed conveniently in two pieces, which then may be assembled around the annular plate 36 and held together, e.g., by bolt 42. 
     A spring 44, e.g., an annular Belleville washer, is pressed between the annular plate 36 and a shoulder 46 formed in the sun gear extension 24 to bias the plate 36 towards the brake pad 28. This in turn presses the brake pad 28 against a brake engagement surface 48 of the sun gear extension 24 to prevent rotation of the sun gear 20 relative to the transmission housing 30. 
     A piston cavity 50 is formed in the sun gear extension 24 on the side of the annular slot 26 opposite from the spring 44, and has an annular piston 52 slidably disposed therein. A hydraulic chamber 54 is defined between the piston cavity 50 and piston 52. The hydraulic chamber 54 can be provided with pressurized hydraulic fluid or drained of fluid in the usual fashion (which is well known to one skilled in the art and therefore will not be described further herein) to press the piston 52 towards the annular plate 36, or to allow the piston 52 to be moved further into the piston cavity 50 by the spring 44. The piston 52 preferably is provided with at least one nose 56 which projects into a corresponding opening 58 in the annular plate 36 to prevent significant relative rotation therebetween. 
     Activation of the piston 52 by provision of pressurized hydraulic fluid to the hydraulic chamber 54 will cause the piston 52 to move the annular plate 36 away from the brake pad 28 and towards the clutch plate 32. This simultaneously will release the engagement of the brake pad 28 with the brake engagement surface 48 and cause the clutch plate 32 to engage the clutch engagement surface 60 of the sun gear extension 24, causing the sun gear 20 to rotate with the planetary carrier 
     The other elements of the planetary shown in the figure, e.g., bearings, lubrication systems and the like, are well known to those of ordinary skill in the art, and therefore will not be described further herein. 
     Various modifications will readily be apparent to one of ordinary skill in the art. For example, while the structure illustrated in the figure provides input to the planetary carrier, draws output from the ring gear and uses a brake and clutch to control rotation of the sun gear relative to the transmission housing and planetary carrier, numerous other connections could be made by one of ordinary skill in the art without deviating from the scope of the present invention. Similarly, while the invention has been shown spring-biased to engage the brake and hydraulically activated to engage the clutch, these features could be reversed as desired for a particular application. 
     While the invention has been described in connection with a specific embodiment, it is to be understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the aforegoing description. Accordingly, this invention is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.